The complement system is a system of plasma proteins which can be activated in the course of the immune response. It is part of the innate immune system. The human complement system consists of more than 30 proteins which are dissolved in the blood plasma or bound to cells. They have the function to protect against micro organisms, however they also have strong cell-destroying properties and may be, if unregulated, responsible for tissue damages in the course of many diseases.
A distinction is made between three pathways via which the complement system is activated, namely the classical pathway which is in most cases induced by antibodies, the lectin pathway which is activated by the mannose binding lectin, and the spontaneous and antibody independent alternative pathway.
A rate determining enzyme of the alternative pathway is the so-called complement factor D (CFD; EC 3.4.21.46) which is also referred to as adipsin or C3 proactivator convertase. CFD is a serine protease which is primarily synthesized in the fat tissue and secreted into the blood circulation. In addition, CFD can be found in different tissues. CFD cleaves the complement factor B. It is described that in the mammal cells the enzyme is synthesized as inactive proenzyme (proCFD) or zymogen and converted into the mature and active form, i.e. the CFD, by cleaving off 5 N terminal amino acids.